Touch probe having nonconductive contact carriers

ABSTRACT

A probe for detecting contact with an object such as a workpiece includes a movable carrier having a first set of contacts mounted thereto and a fixed carrier having a second set of contacts mounted thereon. The first carrier is operable connected to a probe stylus and moves therewith when the probe stylus contacts an object thereby making or breaking electrical connection between one or more of the contacts. Each of the carrier members is made of a body of high flexural strength nonconductive material, preferably glass fiber reinforced plastic, to thereby simplify electrical insulation of the contacts as well as to insure repeatability of the probe measurements.

DESCRIPTION

1. Technical Field

This invention relates to probes for detecting contact with an objectsuch as a workpiece. More particularly, the invention involvestechniques for simplifying the construction of such probes while at thesame time insuring repeatable measurements from such probes.

2. Background

Automated machining systems and coordinate measurements systems requirea precise means for locating surfaces on workpieces. One method ofperforing such measurement is to utilize a so-called "touch probe" inwhich a stylus is moved into a position where it touches the surfaceresulting in a movement of the stylus from its rest position. The probeincludes two carrier members, one fixed and one movable with the stylus,with each carrier member having one or more electrically conductivecontacts mounted thereto and opposing contacts mounted on the othercarrier member. When the stylus moves from its rest position theelectrical characteristics between opposing contacts change. This changein electrical characteristic is typically made by opening the contactsand detecting the resultant change in electrical resistance by suitableelectrical circuitry which transmits a signal to the machine controller.The signal from the probe is used in conjunction with a determination ofthe X, Y and Z axes location of the table or machine spindle tocalculate the position of the inspected workpiece surface.

Touch probes manufactured by the assignee of the present invention haveutilized a set of spherical contacts mounted to a movable carrier memberin the form of a pivot plate connected to the stylus. These sphericalcontacts oppose a second set of contacts presenting flat opposingsurfaces, such contacts being mounted to a fixed member. (See, e.g.,U.S. Ser. No. 388,187, filed June 14, 1982 by Cusack, entitled "TouchProbe", now U.S. Pat. No. 4,451,987.) Other types of touch probesutilize first contacts in the form of radially outwardly extending pinswhich are trapped by two converging spherical surfaces provided by wayof fixed ball bearings (See, e.g. U.S. Pat. No. 4,153,998 to McMurtry;U.S. Pat. No. 4,288,925 to McMurtry; U.S. Pat. No. 4,138,832 to McMurtryand U.S. Pat. No. 4,397,093 to McMurtry).

The usefulness of these touch probes depends upon their capability ofperforming accurate repeatable measurements. It is often desirable toprovide a probe that is capable of repeating its measurement to withinabout one micron or less. In other words, if the probe is used to makeseveral measurements on the same workpiece surface, the calculatedposition thereof cannot deviate more than about one micron.

In probes of this type it becomes necessary to insulate the contactsfrom other electrically conductive material in the probe. Severaldifferent insulation techniques have been employed and some arediscussed in the patent literature identified above. For example, the'823 patent noted above employs components made of synthetic resin towhich the contacts are mounted. However, in order to provide theprecision repeatability properties demanded of a commercially acceptableprobe, most of the probes that are actually sold and used employ metalbodies as carriers for the contacts. While these metal carriers havegood flexural strength, they do present problems in electricallyinsulating the contacts due to their conductive properties. One commonlyused practice is to use anodized aluminum as the carrier members (theanodized layer being nonconductive) and to mount the contacts onto theanodized surface by way of epoxy. Unfortunately, the anodization issusceptable to being scratched or permeated by the epoxy therebyimpairing the insulation properties thereof which can lead to improperprobe operation.

Those skilled in the art appreciate that the manufacture and assembly ofprobes of this type can be a difficult task. Electrical connections mustbe made to the contacts and with the advent of relatively small probeconstructions this can present problems which are not easily overcome.The entire probe must be constructed in such a manner that it is ruggedand can be used over extended periods of time while also insuring thatthe accuracy of the probe measurement is not sacrificed. Thus, it ishighly desirable to provide a probe construction that can be more easilymanufactured while still achieving these high standards.

SUMMARY OF THE INVENTION

In the preferred embodiment disclosed herein, the carrier members forthe contacts are made of nonconductive material having a high flexuralstrength. Preferably, the carrier members are constructed of fiberreinforced plastic, with the fiber content being in the range of 7.5-40%by volume. The high flexural strength of these plastic carriers providesthe probe with excellent performance while at the same time greatlysimplifying the probe construction especially as it relates toinsulation of the contacts.

The overall probe construction employing nonconductive contact carriersis another feature of this invention. One of the carriers takes the formof a pivot plate to which a first set of contacts are inserted. Thesecond carrier is in the form of an insert which is supported on a rigidmetal plate of the probe. A second set of contacts are located on thesecond carrier and oppose the first set when the probe stylus is in itsrest position. The contacts are arranged as a plurality of seriallyconnected switches, the opening of which can be detected by suitablecircuitry. Electrical connection to the switching assembly isaccomplished in a simple but reliable manner. A first electricallyconductive path is established between the detection circuitry and oneof the contacts on the pivot plate. One of the contacts of the secondset is electrically connected to the metallic plate which cooperateswith the probe housing to form a ground plane thereby completing theelectrical connection to the detection circuitry which can be groundedthrough the probe housing.

BRIEF DESCRIPTION OF THE DRAWINGS

The various advantages of the present invention will become apparent toone skilled in the art upon reading the following specification and byreference to the drawings in which:

FIG. 1 is a fragmentary top view of a probe constructed in accordancewith the teachings of the present invention;

FIG. 2 is an exploded perspective view of the switch assembly of thepreferred embodiment of this invention;

FIG. 3 is a simplified end view of the assembled switch assembly asviewed looking towards the right in FIG. 2;

FIG. 4 is a cross-sectional view taken along the lines 4--4 of FIG. 3;and

FIG. 5 is a simplified cross-sectional view showing an alternativeconstruction for connecting the switch assembly to a main probe housing.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In FIG. 1, the techniques of the present invention are shown inconnection with a probe 10 whose main housing 12 is particularlydesigned to be inserted into slots or the like in a turning center suchas a lathe. However, it should be understood that the concepts of theinstant invention may be employed in a variety of different probehousing configurations, for example, such as those adapted to beinserted into the spindle of machining centers. In such case, the probehousing may take other shapes and is generally provided with an adapterfor insertion into the spindle. The instant invention is more directedtowards the switch assembly generally designated by the numeral 14. Theswitch assembly 14 is sometimes referred to as the probe head and may,in some instances, take the form of a separable unit which may beattached to various probe housings as will be described later herein inconnection with FIG. 5.

Probe 10 includes a stylus 16 which is adapted to come into contact withan object such as a workpiece surface. When the stylus 16 is contacted,it is moved from its rest position thereby resulting in a change inelectrical characteristics of the switch assembly 14 which will bedescribed in more detail later herein. This change in electricalcharacteristics is detected by suitable electronic circuitry such ascircuitry 18 which is contained in the main probe housing. Circuitry 18is connected to a power source which, in this embodiment, takes the formof a battery 20. Upon detection of the stylus contact, circuitry 18operates to transmit a signal to the machine controller (not shown) tothereby indicate that the stylus 16 has contacted the workpiece surfaceunder investigation. The signal may be transmitted by various means butone particularly advantageous form is to generate and transmit aninfrared signal via LED 22 to a remote receiver. Suitable transmissionschemes are described in more detail in U.S. Pat. No. 4,328,623; U.S.Pat. No. 4,401,945, U.S. Pat. No. 478,906 by Juengel, filed Mar. 25,1983 entitled "Apparatus for Detecting The Position of a Probe Relativeto a Workpiece;" and co-pending U.S. Ser. No. 501,994, filed June 14,1983, entitled "Turning Tool Probe", each of which are assigned to theassignee of the present invention. These patent and applications arehereby incorporated by reference. However, a wide variety of othertransmission schemes are within the scope of the instant invention.

Special attention should now be given to the construction of the switchassembly 14. An important aspect of this invention is that the contactcarriers 24 and 26 are made of high flexural strength nonconductivematerial. Carrier member 24 is coupled to probe stylus 16 and is movabletherewith; carrier 26 is fixed. In this embodiment, carrier 24 takes theform of a triangularly shaped pivot plate having three equally spacedcontacts in the form of ball pins 28, 30 and 32 mounted thereto in themanner to be described. Opposing this first set of contacts is a secondset of fixed contacts taking the form of the discs 34, 36 and 38 mountedto carrier 26.

When the stylus 16 is in its rest position the balls 28-30 are urgedagainst their respective discs 34-38. Each of the three ball-disc pairscomprise a switch and these three switches are electrically connectedtogether in series as will be described. The contacts are held inalignment by way of a reed spring 40 having a tab 42 connected to pivotplate carrier 24 via retainer block 44 and screws 46, 48. The peripheryof reed spring 40 is sandwiched between housing 12 and a metallicsupport plate 50. The switch assembly 14 is protected from adverseenvironmental conditions by way of a diaphragm 52 nd O-ring 54 which areheld in place by an outer plate 56. Outer plate 56 is attached tosupport plate 50 by way of screws 58, support plate 50 being similarlyconnected to the metallic main probe body 12 by way of screws 60.

When stylus 16 contacts an object, it is displaced from its restposition thereby tilting pivot plate carrier 24 and lifting at least oneof the ball contacts 28-32 from its respective disc contact 34-38. Thisprocedure opens one of the ball-disc switches and is sensed by circuitry18 causing it to transmit a signal via LED 22.

It can be appreciated that probe constructions of this general typenecessitate that the electrically conductive contacts be electricallyinsulated from other conductive probe components. In addition, it isimperative that the carriers for the contacts have very high flexuralstrength with minimal yielding properties in order to insure properprobe operation. The carriers should exhibit flexural strengthspreferably exceeding 20,000 psi (120 newtons/mm²). It has beendiscovered that by making the carriers 24 and 26 out of fiber reinforcedplastic that both of these constraints can be met.

Carriers 24 and 26, in this embodiment, are injection molded partscomprising glass fiber reinforced plastic resin bodies. The ratio offibers to resin should be high enough to provide the required flexuralstrength while not being so high as to cause the part to become brittleor cause problems in the molding process. The glass fiber content shouldbe 7.5-40% by volume, with a fiber content of about 30% by volume beingpresently preferred.

Various types of resins may be utilized but poly(butyleneterepthalate)known as PBT has provided excellent results. One commercially availablereinforced plastic resin meeting the above constraints is the 400 familyof Valox resins, preferably Valox-420, available from General ElectricCompany.

Special attention should now be focused on the method by which thecontacts are mounted and the way in which electrical connection is madethereto. The contacts 28-32 consist of tungsten carbide balls brazed toberyllium copper shanks 64. As can be seen most clearly in FIG. 4, theshanks are inserted through holes formed in pivot plate carrier 24. Theend of shank 64 includes a circumferential groove 66. An arcuate shapedconductive foil 68 includes openings at each end that are snap fit overthe grooves 66 in the shanks of contacts 28-30 to provide electricalconnection therebetween. A second foil 70 (see FIG. 2) surrounds a neck73 on carrier 24 and is similarly snap fit over the end of the shank ofcontact 32. The contacts are firmly held in place by way of conductiveepoxy 72 and 74 on both sides of the carriers 24 as can be seen mostclearly in FIG. 4. A coil spring 67 engages foil 70 and serves the dualpurpose of urging pivot plate carrier 24 towards fixed carrier 26 and tomake electrical connection between circuitry 18 in the probe housing 12and the switch assembly 14.

Turning now to the fixed carrier 26, it takes the form of a C-shapedannulus which fits within and is supported by a well 80 formed withinsupport plate 50. The outer surface of carrier 26 includes two arcuateslots 82 and 84 formed therein. Slot 82 is larger and is designed tocontain both the discs 34 and 36 at its outer ends. A conductive foil 86lies within slot 82 and the discs 34 and 36 are placed thereon. Therecessed areas between discs 34 and 36 is filled with conductive epoxyto hold the discs in place.

A foil 88 lies within the smaller slot 84. One end of foil 88 includesan opening 90 which becomes aligned with openings 92 and 94 in carrier26 and support plate 50, respectively. A conductive pin 96 passesthrough openings 90-94 with the head of pin 96 being in contact withfoil 88 and the shank of pin 96 being in electrical contact with plate50. This is shown most clearly again in FIG. 4. Disc 38 is disposed onthe other end of foil 88 and conductive epoxy 98 holds the subassemblyin place.

The construction noted above simplifies the electrical connection to theplurality of ball-disc switches in a manner which insures goodelectrical integrity. The electrical connection is best explained inconnection with FIGS. 1 and 2. The negative side of battery 20 isconnected to the metallic probe housing 12 via a conventional springclip 19. The positive battery terminal is connected to circuitry 18.Circuitry 18 in probe housing 12 is connected by way of a wire, cable,or the like (represented by the line 21 in FIG. 1) to coil spring 67 ofthe switch assembly 14. The opposite end of coil spring 67 contacts foil70 connected to ball contact 32. Ball contact 32 rests on disc 36thereby forming the first switch. Disc 36 is electrically connected byway of foil 86 to disc 34. Disk 34 opposes ball contact 28 therebyforming the second switch. The shank of ball contact 28 is connected byway of foil 68 to the shank of ball contact 30. Ball contact 30 opposesdisc 38 thereby forming the third switch, each of the switches beingelectrically connected in series. Disc 38 is connected to the metallicsupport plate 50 by way of foil 88 and pin 96. Since plate 50 ismetallic and is physically connected to the metallic probe housing 12,it forms a ground plane completing the electrical loop back to thebattery 20 and circuitry 18 in the probe housing.

FIG. 5 illustrates a presently preferred probe construction whichemploys a separable head 100 containing the switch assembly. Thisconstruction uses the same basic components as described above and theselike components will be referred to by the same reference numerals withprimed superscripts added. The main probe housing 12' is metallic and isdesigned to have the batteries 20', circuitry 18' and one or more LED's22' mounted in the general locations shown. One end of housing 12'includes an internally threaded annular ferrule 102 having suitableelectrical connector means 104 therein coupled to circuitry 18'.

Head 100 is metallic and includes threads on one end which allow thehead to be screwed into ferrule 102. Means 104 thus makes electricalconnection with coil 67' and the connection is made to the threeball/disc switches as described above. The support plate 50' isconnected to the metallic head housing which completes the groundcircuit back through ferrule 102 in the main probe housing 12'. The useof the separable head 100 permits the same switch assembly constructionto be used with a variety of different probe housings, each housingbeing particularly adapted to fit a given machine. For example, theelongated circular battery compartment 12' of the construction of FIG. 5is designed to fit within holders for boring bars whereas therectangular cross section of the housing 12 of FIG. 1 is primarily usedin slots in turrets.

Regardless of the type or probe housing or signal transmission schemeemployed, the construction of the switch assembly as defined by thefollowing claims provides the probe with excellent measurementcharacteristics while also simplifying its construction.

I claim:
 1. In a probe with a stylus for detecting contact with anobject, the probe having a switch assembly including a first carriermember transversely connected to the stylus and having a first set ofelectrically conductive contacts affixed thereto, a second carriermember having a second set of electrically conductive contacts affixedthereto, and whereby electrical characteristics between the two sets ofcontacts are changed when the stylus contacts an object, wherein theimprovement comprises:at least one of said carrier members beingconstructed of a body of electrically nonconductive fiber reinforcedplastic thereby simplifying electrical insulation of the contactsaffixed thereto while insuring repeatable accurate measurements of theprobe.
 2. The improvement of claim 1 wherein said body is constructed ofglass fiber reinforced plastic resin containing 7.5-40% by volume glassfibers.
 3. The improvement of claim 2 wherein said plastic resin ispoly(butyleneterephthalate).
 4. The improvement of claim 1 wherein saidfirst carrier member comprises a fiber reinforced plastic pivot plateconnected to the stylus, and wherein said second carrier membercomprises a fiber reinforced plastic insert supported on a fixedmetallic plate.
 5. The improvement of claim 4 wherein each of thecontacts of said first set includes an elongated shank terminating atone end in a spherical-shaped configuration and having a groove locatedat an opposite end of the shank; and wherein electrical connectionbetween two of the contacts of the first set is made by way of a foilpress fit onto the grooves of the shanks of two of the contacts.
 6. Theimprovement of claim 5 wherein said insert includes at least onearcuate-shaped slot formed in a surface thereof, and wherein contacts ofthe second set comprise discs, with two of the discs being mounted insaid slot.
 7. The improvement of claim 6 wherein a third disc contact ofthe second set is located in a second slot formed in the insert, andwherein said second slot includes means for electrically connecting thethird disc to the metallic plate.
 8. The improvement of claim 7 whereinsaid means comprises a second foil lying in said second slot, and anelectrically conductive pin coupled at one end to the foil and having ashank portion passing through the insert into the metallic plate.
 9. Ina probe for detecting contact with an object, the improvementcomprising:a pivot plate constructed of fiber reinforced plastic, aplurality of first ball contacts each having a shank terminating in aball at one end which is located on one side of the pivot plate, withthe shank passing through the pivot plate and having an opposite endlocated on the other side of the pivot plate, first means forelectrically connecting together said opposite shank ends of two of thefirst contacts; a stylus connected to the pivot plate; a fixed metallicsupport member; an insert constructed of fiber reinforced plasticsupported by the support member, said insert including a plurality ofsecond disc contacts thereon each presenting flat surfaces for opposingthe balls on the pivot plate when the stylus is in a rest position,second means for electrically connecting one of the second contacts tothe metallic support member; and third means, including said first andsecond means, for electrically connecting each pair of ball/disccontacts into a series circuit whereby displacement of the stylus fromits rest position results in a change in electrical characteristicbetween at least one ball/disc pair.
 10. The improvement of claim 9wherein said first means comprises a foil press fit onto the ends of theshanks of two of the first contacts.
 11. The improvement of claim 9wherein said insert includes a first slot formed in a surface thereof,with two of the discs being located within the slot and electricallyconnected together by fourth means lying in the slot.
 12. Theimprovement of claim 11 wherein said fourth means comprises a foil lyingin the slot upon which the discs rest.
 13. The improvement of claim 12wherein said insert includes a second slot formed therein, with a thirddisc being located in the second slot, and wherein said second meanscomprises a pin passing through the insert, said pin having one endelectrically connected to the third disc and its opposite end contactingthe metallic support member.
 14. The improvement of claim 13 whereinsaid second slot in the insert further includes a third foil on whichthe third disc rests and which is electrically connected to said pin.15. The improvement of claim 14 wherein said first and second slots inthe insert are filled with conductive epoxy.